Within the International Space Station (ISS) oxygen generator, an increase in differential pressure across a pump supplying return water to a PEM electrolyzer fuel cell stack had persisted over a 4-month period and was approaching the shut-off limit for the system. This decrease in performance was suspected to be caused by water-borne catalyst fines containing platinum black and Teflon®* binder materials, shed by the fuel cell stack, and accumulated within the pump's inlet filter. Maintenance in the field was required.
The system had been designed for factory maintenance, and no contingency had been planned to handle field maintenance for such a circumstance. An initial assessment of hazards for the proposed filter maintenance raised the concern that opening the water line would release hydrogen gas from pressurized water, and with the sudden exposure of the debris collected on the filter to oxygen from outside, put both fluids in contact with the catalyst fines on the filter. Dry catalyst is known to heat to temperatures over 800 °C when exposed to hydrogen and oxygen. This could pose a fire hazard to personnel and surroundings. In addition, exposure of the Teflon binder to high temperatures can produce micron-scale Teflon particles, hydrogen fluoride (HF), and carbonyl fluoride (COF2) that pose an acute toxic inhalation hazard. However, water in the system will mitigate heat generation by a combination of heat sinking and physically limiting the availability of oxygen to approach the catalyst quickly. The factors that would prevail were not known. No action could be taken until these potential hazards were better understood.
* Teflon® is a registered trademark of E.I. DuPont de Nemours and Company, Wilmington, Delaware.
- Hydrogen Production/Use Systems
- Water Electrolysis System
The possible outcomes from new maintenance scenarios can be predicted by using an accurate simulation. The proposed filter change-out maintenance was studied to identify conditions to which the catalyst might be exposed and a mock-up of the filter, it's mounting/housing, and catalyst was assembled. Conditions selected to represent the worst case that could be encountered during a maintenance operation were reproduced. For the conditions of temperature and pressure, this included simulating the state of the catalyst. Oxygen was removed from the catalyst with a dry nitrogen purge, and followed by a graduated hydrogen purge from 0.5 to 10 % concentration. Hydrogenation was completed by then subjecting the catalyst to a purge of 100 % concentration of hydrogen. De-ionized water was vacuum-degassed for 10 minutes to remove oxygen, then saturated with hydrogen (by bubbling) at pressure. The simulation of pre-change-out filter conditions was completed by adding the hydrogen-saturated water to the hydrogenated catalyst within the mock-up system. The simulation of conditions introduced by the proposed maintenance was accomplished by draining the water and introducing a purge of oxygen-enriched air (ISS ambient conditions). The drained water and purged air were captured for laboratory analysis to check for thermal degradation and toxic byproducts of Teflon. Gas Chromatography with Mass Spectral detection (GC-MS) revealed no fluorinated species above the detection limits in the gas phase, and Ion Chromatography (IC) identified only small amounts of fluorinated compounds in the liquid phase (not indicative of a handling hazard). Post-test thermal gravimetric analysis (TGA) of samples indicated that insufficient heat was generated in the tests to thermally decompose the Teflonized catalyst. The conclusion is that when charged catalyst is handled wet, reactions with air are reduced to the point of permitting safe handling. A caution is noted. While deliberate simulation of conditions that would result in rapid exposure to dry charged catalyst was not performed, the hydrogenation step, if not initially done gradually, but with 100 % concentration hydrogen, will produce smoke and steam, suggesting any procedure that introduces rapid exposure to air could result in high temperatures and potentially hazardous by-products.
Maintenance on PEM technology systems that involves accumulated catalyst within filters may be safely performed by reducing the rate of exposure of catalyst materials to air or oxygen, always keeping a coating of water on catalyst surfaces and making sure discarded catalyst is properly disposed and not allowed to dry out, especially in the presence of volatile and flammable materials. Catalyst materials exposed for a time in a hydrogen environment will absorb substantial amounts of hydrogen, becoming "activated". Subsequent exposure to air or oxygen without mitigating steps can cause high temperatures to occur on catalyst surfaces, creating a potential fire hazard. Do not expose Teflon to high temperatures.